Dark matter is the subject of an investigation led by teams of scientists on the biggest case of the 21st century. It is a phenomenon whose observation was induced by the movement of stars further away from the galactic center, revolving faster than the ones closer in. In the cold dark matter cosmology, stars and gas are thought to be mixed with and embedded in dark matter. In the local Universe, its mass within a galactic disk increases with disk radius, becoming appreciable and then dominant in the outer, baryonic regions of the disks of star-forming galaxies.
Nothing seems clear cut in this old mystery. A lilliputian telescope with dragonfly eyes, invented by Pieter van Dokkum from Yale University, discovered the Dragonfly galaxy 44 with an extraordinary concentration of dark matter and identified another galaxy which has far less dark matter than expected, and perhaps no dark matter at all. Its missing matter begs the question as to whether this “missing dark matter problem” is unique to this galaxy or applies more widely.
Luminous or non-luminous, matter can either be seen with the right instruments or stay in the dark. Vera Rubin wrote that “the (dark) matter could be much like the matter we know” but somehow “it is not radiating” or it is a “kind of particle that we have not yet discovered, and which does not radiate.” (Bright Galaxies, Dark Matters, Springer Science & Business Media, 1996, p.206). As invisible as dark matter is, scientists are retracing its evolution. A pair of extremely massive star-forming galaxies at less than 800 million years after the Big Bang has been discovered, suggesting the presence of a dark-matter halo with a mass of more than 400 billion solar masses, making it among the rarest dark-matter haloes that should exist in the Universe at this epoch.
The fundamental nature of dark matter constitutes ∼ 85% of the matter density and ∼ 26% of the energy density of the universe. After cosmic inflation in the early universe, a strong gravity has caused most of matter to coalesce into small impenetrable closed systems interacting only by gravity and constituting most of the dark matter. Some studies have raised the possibility that it is composed of primordial black holes while others have ruled them out as the dominant form of dark matter. Others have suggested that it is formed by remnants of evaporated black holes tunneling to white holes through which matter bounces back. The term “erebons” coined by Roger Penrose describes those dark matter particles crossing over from one eon to the successive one across “dark epochs” in cyclic cosmologies.
Scientists have gained a better understanding over the past 80 years thanks to their increasing abilities to map the distribution patterns of dark matter. From the tens of thousands of galaxies mapped out by the Swiss astronomer Fritz Zwicky to the millions of galaxies whose images are being gathered today, they have been able to study the multi-faceted phenomenon but remain somehow helpless at assessing its physical reality. Following the two- and three-dimensional mass maps reconstructed from the Subaru Hyper Suprime-Cam survey first-year shear catalog, the Large Synoptic Survey Telescope will be able to produce the largest and most detailed map of the distribution of matter and the growth of cosmic structure over the past 10 billion years.
I don’t know what’s more exciting about the discussion on dark matter if it’s the fact that it is yet another mystery that the human mind is set out to decipher or that countless of scientists rely on an abundance of technological feats in their quest. Optical imaging surveys and detectors in South Korea (KIMS,COSINE-100), China (PANDAX), Canada (PICO, SuperCDMS ), Italy (Xenon 100, DAMA/LIBRA), France (Edelweiss), Japan (XMASS), United States (LZ, LUX ) represent just one aspect of the collaborative undertaking in the search for answers.
Is dark matter a fluid or a particle? A study found that dark matter is a cold, collisionless, fluid that can be kinematically ‘heated up’ and moved around. Several new types of fundamental particles have been proposed as candidates for dark matter but no definitive signal has been detected except by the DAMA collaboration which reported a statistically significant annual modulation in the rate of interaction events in their detector with a period and phase consistent with that expected for weakly interacting massive particles (WIMPs). However, the result of the first 59 and half days of data from COSINE-100 experiment ruled out WIMP-nucleon interactions as the cause of the annual modulation observed by the DAMA collaboration. Some other studies suggest that massive pseudo-Goldstone bosons named axions, interacting with photons, are the plasma particles that are the most probable candidates for dark matter particles. Yet, others claimed that the massive gravitons, linked to the generation of massless gravitons, i.e. the gravitational waves, are a possible candidate. Furthermore, sterile neutrinos are found to be a theoretically very well-motivated dark matter candidate that can be searched for indirectly and directly.
While some are figuring out what dark matter is made of, others are striving to get around it and redefine the laws of gravity. A pilot study on the basis of the individual measurements of galaxies and galaxy clusters is aimed at putting into test if gravity can indeed be modified at large scales, and what this would imply for such observables as the mass density of dark matter. Furthermore, Erik Verlinde is making the case that the observed phenomena that are currently attributed to dark matter are the consequence of the emergent nature of gravity, an unavoidable and logical consequence of the emergent nature of space time itself.
The debate on the luminous and non-luminous matter is reminiscent of a similar discussion about Consciousness and the Unconscious mind. The Unconscious seems to nurture our conscious thoughts and push us unaware on our life journey the same way, it appears to me, dark matter is said to influence the stellar motion. As we wonder whether the Unconscious is outside the realm of consciousness or an inaccessible part of it, we ponder whether dark matter is made of a “kind of particles that we have not yet discovered" or “much like the matter we know”. In that context, I recall that some have imagined the possibility of a biological dark matter and conjectured that consciousness can be understood as yet another state of matter.